The lighting industry has been witness to an explosion of the number of types of commercially available fluorescent lamps. Whereas twenty years ago there were between 40 and 45 lamps available, today there are over 300 different types of fluorescent lamps available on the market. Each type of fluorescent lamp has its own set of uniquely rated characteristics, such as running voltage and filament impedance. In order to properly start, run and dim a lamp, these characteristics must be carefully taken into account. Accordingly, lamp characteristics must be painstakingly matched with the appropriate ballast to avoid lamp failure.
While it is easy to physically replace one fluorescent lamp with another fluorescent lamp of a lower wattage (for example the T12 with a T8) or of a different type but with the same or similar wattage (for example the T8-18W and the PLC-18W), simply doing so can cause serious problems. First, since every ballast design is optimized for a particular lamp with a particular set of characteristics, a lamp of a lower wattage will not usually start reliably in a ballast designed for a higher wattage lamp. Second, operating characteristics such as filament impedance and the lamp current are normally substantially different for different lamp types, which will result in asymmetric and distorted lamp voltage and current waveforms causing considerable lamp flicker. Finally, the different operating characteristics of a lower wattage lamp, for example, can cause a larger rms current to be drawn from the ballast. This results in lamp current easily exceeding the rated ballast load current and leads to early ballast failure.
As a result, ballast manufacturers are forced to carry increasing inventories of ballast types as lamp manufactures continue to develop new lamp types. It is common industry practice for ballast manufacturers to routinely stock hundreds of different ballast configurations in order to comply with the conditions of lamp warranties. Further, the production cycle and the full market value of a new fluorescent lamp technology is dependent on the presence of a corresponding ballast, built to accommodate the new lamp's operating characteristics. Delays in the production of lamp-specific ballast equipment causes systemic market and production inefficiencies which are not easily resolved even through strategic planning or industry cooperation.
Accordingly, it has been the aim of many ballast designers to design a ballast which can accommodate various types of gas discharge lamps without the need to physically alter the ballast's hardware configuration.
Ballast designers have designed adaptor circuits which can be used to retrofit ballasts so that one type of lamp can be safely replaced by another. U.S. Pat. No. 4,701,673 to Lagree et al. discloses such a device which converts a conventional two lamp rapid start T12 ballast into a ballast that will operate two T8 fluorescent lamps. The adaptor circuit comprises an auxiliary circuit including a tuned series-parallel LC network connected in parallel with one or both of the lamps and tuned to supply an odd harmonic current to the lamps. While such a solution allows two different types of lamps to be accommodated by a particular hardwired ballast, such devices can only offer modest retrofitting capability as they can only accommodate a small number of lamp types and require the installation of external circuitry.
Another approach has been to design ballasts that provide variable current to a lamp by varying the frequency of the inverter circuit. U.S. Pat. No. 5,287,040 to Lestician describes such a ballast which uses isolation transformers, operating in their "high frequency zone" to feed power to one or more fluorescent lamps. An increase in frequency (with voltage held constant) will cause a decrease in output current and thus by appropriately setting the nominal operation frequency of the transformer, different lamp sizes can be accommodated without rewiring or changing components.
The range of lamps which can be accommodated using this technique is limited due to the fact that the inverter frequency must be confined within the range of 20 and 55 KHz to meet FCC ballast operational standards. This range is further limited due to informal industry recommendations that inverter frequencies not exceed 47 KHz in order to avoid interference with television remote control devices. Further, the frequency of the pulse signal used to drive the circuit cannot fall below a critical threshold frequency i.e., the loaded resonant frequency. Below this threshold, the circuit begins to oscillate in a "capacitive" mode, leading to destruction of circuit components. In addition, circuit components do not exhibit optimal performance throughout the range of frequencies that may be needed for control.
Finally, some ballast designers have used microcontrollers to adjust lamp current according to stored lamp loading data as in U.S. Pat. No. 5,039,921 to Kakitani which adjusts the frequency of the inverter to change lamp voltage. Again, while this invention provides for the adaption of the ballast to various types of gas discharge lamps, the range of lamps which can be accommodated using frequency control is limited due to allowable frequency range which may be used and other circuit performance factors. Further, other critical operational factors, such as starting and dimming are not contemplated.
Another ballast design dealing with dimming is exemplified by U.S. Pat. No. 5,583,402 to Moisin et al. which describes an inverter control circuit that is used to adjust the duty cycle or frequency of an inverter signal to change the level of current flowing through the lamp. The lamp is connected into a resonant circuit, tuned such that a change in the duty cycle of the AC signal changes the level of current flowing through the load. However, due to the low Q factor of the resonant circuit, the change in frequency only has a minor effect on the dimming of the lamp. Further, this inverter control circuit is directed to providing variable current to a lamp for dimming, without regard to other factors critical to the operation of a lamp, such as running or starting conditions.
For a ballast to have practical universal application to a wide range of lamp types, it must be able to appropriately start, run and dim a lamp according to that lamp's particular characteristics. It is also desirable for such a ballast to provide superior starting and dimming functionality using cost effective components.
Starting circuits are often unreliable due to various environmental conditions such as static discharge. Further most lamp striking circuits do not comply with long established ANSI standards. Dimming circuits for use with gas discharge lamps are typically complex, requiring a high number of components and making them expensive to build, install and retrofit to existing ballasts. Further, most prior art fluorescent dimmers can only achieve dimming rates for compact fluorescent lamps of approximately 25% and approximately 10% for linear fluorescent lamps using variable frequency methods. While some manufacturers have attempted to improve the dimming range by changing the duty cycle of the inverter signal, these methods are notoriously unreliable as they often result in a loss of the plasma thread causing the lamp to extinguish. It is believed that this occurs because the capacitance conventionally connected across the lamp passes the high frequency harmonics which comprise much of the energy in a low duty cycle signal, thereby shorting the lamp.
Thus, there is a need for a universal lighting ballast which is suited to operate a wide range of different fluorescent lamp types, and which can offer improved dimming and starting functionality on a cost effective basis.